Why did AMD announce Mantle?
What's really odd is that AMD must have known DX12 was coming down the line as it announced Mantle, but if DX12 delivers on Microsoft's claims, you have to wonder what the point of Mantle is.
OK, Mantle may be even more efficient at getting beneath those performance-sapping abstraction layers, but it's also surely a case of diminishing returns. And more to the point, Mantle is an AMD-only where DX12 includes AMD, Nvidia and Intel GPUs on the PC and yet more in the mobile space.
That's a huge advantage when it comes to the critical game developer-support angle. Unless Mantle has a spectacular technical advantage over DX12, it's very likely dead in water.
Like Mantle, DX12's true potential is untested so perhaps AMD knows something that we don't and that's why it pressed ahead with Mantle. Time will tell.